php mailing list operation

Hi,
I'm using the mail() function to send out newsletters to subscribers,
problem is that it seems a little unreliable...
at the moment I assemble the mail outside a loop then iterate through
all the mail addresses
for($i=0;$i<$numaddresses;$i++){
mail($to[$i],$subj,$message,$fromstr);
}
should i be putting in a delay message or a retry_on_fail method or
anything?

Martin

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